1. Field of the Invention
The invention relates to operations in TDD (time division duplex) mode in third generation cellular telecommunication systems. Especially, the invention is related to such a method as specified in the preamble of the independent method claim.
2. Description of Related Art
With reference to FIG. 1, a typical third generation mobile telephone system structure will be shortly described. Only those functional blocks are shown which have certain importance to the description of the present invention; it is obvious to a person skilled in the art that a common mobile telephone system also comprises other functions and structures, which need not be discussed in greater detail here. The main parts of the mobile telephone system are: a CN or a core network 101, a UTRAN or UMTS terrestrial radio access network 102 and a UE or user equipment 103. The user equipment is often called a mobile terminal or a mobile communication means. The interface between the CN and the UTRAN is called the Iu interface, and the interface between the UTRAN and the UE is called the Uu interface.
The UTRAN is composed of RNSs or radio network subsystems 104. The interface between two RNSs is called the Iur interface. The RNS comprises a RNC or radio network controller 105 and one or more node Bs 106. The interface between an RNC and a node B is called the Iub interface. Each node B gives rise to at least one coverage area, i.e. cell, which is designated in FIG. 1 by 107.
Presently the development of third generation cellular systems is headed towards the use of a plurality of communication techniques and transmission modes over the air interface. For example, according to current plans, conventional GSM mobile phones can be used also with third generation cellular systems via GSM-capable radio access networks (UTRAN). Many other transmission modes will also be supported. The present application concerns the UTRA (UMTS Terrestrial Radio Access) TDD (time division duplex) mode.
In the UTRA TDD system all physical channels have the structure of radio frames and timeslots. The frame has a duration of 10 ms and is subdivided into 15 time slots (TS). A time slot corresponds to 2560 chips. The time slots separate different user signals in the time domain, and several bursts can be sent in the same slot separated by differing spreading codes. Each of the time slots can be allocated to either the uplink or the downlink. The allocation can be nearly symmetric or even highly asymmetric, if needed. At least one time slot has to be allocated for the downlink and at least one time slot has to be allocated for the uplink. The flexibility in the allocation of time slots in uplink and downlink directions allows the TDD mode to be adapted to highly differing environments.
The data symbols of a channel is sent in bursts. One burst is transmitted in a time slot. FIG. 2 shows the structure of a burst. A burst comprises a first data part, a midamble, a second data part and a guard period. Two types of bursts are defined in UTRA TDD, which have different lengths for the data parts and the midamble. FIG. 2 shows the lengths of a type 1 burst, data parts being 976 chips long and the midamble 512 chips long. The midamble of a type 2 burst is 256 chips long, and the data parts 1104 chips long. The guard period in both types of bursts is 96 chips long. The midamble of a burst carries no payload data. The midamble functions as a training sequence for use by a receiver in signal acquisition and tracking.
The UTRA TDD mode is described in more detail in various 3GPP (3rd Generation Partnership Project) specifications, such as the TS 25.221 V3.0.0 specification describing physical channels and mapping of transport channels onto physical channels.
In UTRA TDD the paging mechanism uses two channels, namely the PICH (Paging Indicator CHannel) and the PCH (Paging CHannel). Paging messages are carried in the PCH, and the PICH carries only indications, if paging messages relating to mobile terminals in a given paging group are to be expected. The paging channel is transmitted over a paging area (PA) which may comprise one or more cells. The number of mobile terminals within the paging area can be large, whereby the traffic volume of the paging channel can also be large. This means that if a mobile terminal were to receive paging messages only with the help of the PCH, the mobile terminal would have to listen to paging messages of PCH, which would consume an excessive amount of time for the terminal. This would cause excessive power consumption especially during sleep mode, in which the power consumption of mobile terminals should be as low as possible. The paging indicators carried in PICH indicate to each paging group of mobile terminals, if a paging message is to be expected on the PCH to some terminal of the paging group. Receiving of a paging indicator requires only receiving of a single burst and decoding the data bits of the burst to obtain the paging indicator values. The paging indicators are sent at predetermined intervals, so that terminals can stay in sleep mode between the paging indicator bursts. If a paging indicator indicates that a paging message is to be expected for the paging group of a particular mobile terminal, that mobile terminal begins to listen to PCH for a certain period to find out, if any paging messages are intended to that mobile terminal. Terminals in a paging area are divided into 60 paging groups, which allows the number of mobile terminals to stay rather low. The low number of terminals in a paging group results in a low number of paging messages intended for terminals in the group, whereby the terminals need to listen to PCH only for short periods of time. This results in a low power consumption in the sleep mode.
However, a mobile terminal also needs to monitor the quality of the radio link to the current cell of the mobile terminal. If the quality of the radio link degrades for example due to moving of the terminal out of the coverage area of the cell, the mobile terminal needs to perform a handover to another cell. For this purpose, a mobile terminal needs to receive a burst in the PCCPCH (Primary Common Control Physical Channel) and measure the reception level of the midamble of a burst carrying PCCPCH information. If the reception level is too low, the terminal needs to start searching and listening to neighboring cells to find a cell, whose transmissions the terminal can receive at a sufficient level.
Although the use of PICH already optimizes the sleep mode operations rather well, reception of PICH bursts and monitoring the strength of the PCCPCH channel still requires a terminal to leave the sleep state quite often. The prior art does not teach any way to further reduce the necessity to receive and consequently the power consumption of the mobile terminal.